Marcelo's injury exposes Zinedine Zidane's lack of left-back options

Luka Modric rightly took the plaudits after Real Madrid's battling 2-1 win over a Granada side that, on the evidence of Sunday's performance, should be a lot further from the bottom of the Liga pile than they are.

The Croatian has evolved at the Bernabeu to the extent that his should be -- if it isn't already -- the first name on the team-sheet. That Zinedine Zidane celebrated Modric's goal with gusto is no surprise.

Victories for Barcelona and Atletico meant the pressure was on to dig out a result in Los Carmenes. But there was one sour note among the euphoria. Marcelo left the stadium with his shoulder in a sling and will undergo tests on Tuesday to determine the extent of his injury. Although nothing had been determined by Monday evening, the worry is that the Brazilian full-back may have suffered a dislocation.

A quick glance at the bench from Sunday's game reveals the extent of Zidane's dearth of defensive options. Nacho and Alvaro Arbeloa were the only reinforcements available to the Frenchman who, along with everybody else at the club, will be hoping that Marcelo's injury will not be as serious as the one that has sidelined Sergio Ramos for large parts of the season.

Real's board rolled the dice in deciding to postpone any new signings until the summer, based on a conviction that the club's transfer ban would be suspended on appeal to FIFA. In the meantime, there are few areas of the squad that look light for a Liga and Champions League challenge, but left-back remains a glaring deficiency.

Real let Fabio Coentrao leave in the summer after a long period of injury problems and largely underwhelming performances, but the Portugal international was at least a reliable option when he was available. It would be a push to describe any of Zidane's alternatives to Marcelo to be anywhere near that status.

Danilo put in a few encouraging performances under Rafa Benitez when Marcelo was unavailable, but Zidane has shown no inclination to give the Brazilian another run-out after his display at Betis. Danilo was on the bench for the next match against Espanyol and didn't even make the squad for the trip to Granada.

That leaves either Nacho -- who has been a bit-part player at best during his Real career and last appeared at left-back in the 3-2 loss to Sevilla in November -- or Arbeloa. Since Zidane took over, Nacho has played four minutes of football. Arbeloa has played four minutes fewer than that and has only made the bench twice since a brief cameo in the 10-2 win over Rayo before the Christmas break. It's not a very encouraging panorama with key games against Athletic Bilbao and Roma coming up in the next week.

Athletic will at least travel to the Bernabeu without Inaki Williams, who was sent off at the weekend. The sight of one of the contenders for young player of the year running at Arbeloa, surely a candidate for quite the opposite accolade, would not be an encouraging prospect for Madrid fans.

Marcelo has been in good form in his 20 La Liga appearances for Real Madrid this season.

Even so, Ernesto Valverde will be rubbing his hands at the prospect of playing Iker Muniain in Williams' place with Sabin Merino on the left. The absence of the injured Raul Garcia is also a boost for Real, but Aritz Aduriz is quite capable of tying two or three defenders down on his own. It's an immediate concern for Zidane, but of much greater concern is the trip to the Olympic Stadium.

Roma have lost only once under new coach Luciano Spalletti, who has shown a willingness to tinker with formations since his arrival. That loss was to Juventus, in Turin, and by a 1-0 score-line.

On that occasion, Spalletti fielded a 3-4-1-2, sacrificing possession to the counter-attack. It's something Zidane could consider against Athletic, if he has Gareth Bale and Pepe available: Sergio Ramos as a sweeper with the Portugal defender and Raphael Varane either side, Modric and Toni Kroos controlling the midfield and Bale and Dani Carvajal acting as wing-backs.

Bilbao are short of wide players with Ibai Gomez a long-term injury absentee, and it could serve as a blueprint to counter whatever Spalletti dreams up for Real's visit to the Eternal City next week. Bale destroyed Inter Milan in 2010-11 from the left of midfield and is quite capable of putting in a shift in defence, as well.

Ronaldo and Karim Benzema could fill the central positions with Isco or James in the hole. Real remain a more potent threat on the counter than playing the possession game, and with Marcelo's propensity to get forward, the team effectively operate with three at the back and a covering midfielder for long periods.

Zidane displayed a bit of tactical flexibility at Castilla, trying out at various times 4-3-3, 4-2-3-1, 4-4-2 and 4-1-4-1. He fitted the system to the players he had at his disposal. If he doesn't have the personnel in the potential absence of Marcelo, he could decide to go further.

In all reality, Zidane won't. He'll just be left to rue the board's reluctance to address a squad issue that has been apparent for several seasons and could have been fixed just as many times, not least in January. Both Valverde and Spalletti will be concentrating their own tactical efforts on Real's left if Marcelo is a confirmed absence. That responsibility lies with the board, not the manager.

Whatever else he has on his plate on Tuesday, Zidane will be keeping an eye on the Sanitas clinic in Madrid's La Moraleja neighbourhood, where Real's players go to get fixed. And he'll be hoping that Marcelo emerges free of the sling he left Los Carmenes with.

Rob Train covers Real Madrid and the Spanish national team for ESPN FC. Twitter: @Cafc13Rob.